Apple DOS was the family of disk operating systems for the Apple II series of microcomputers from late 1978 through early 1983. Apple DOS had three major releases: DOS 3.1, DOS 3.2, and DOS 3.3; each one of these three releases was followed by a second, minor "bug-fix" release, but only in the case of Apple DOS 3.2 did that minor release receive its own version number, Apple DOS 3.2.1. The best-known and most-used version was Apple DOS 3.3 in the 1980 and 1983 releases. Prior to the release of Apple DOS 3.1, Apple users had to rely on audio cassette tapes for data storage and retrieval, but that method was notoriously slow, inconvenient, and unreliable.

Apple DOS 3.3 was released in 1980. It improved various functions of release 3.2, while also allowing for large gains in available floppy disk storage; the newer P5A/P6A PROMs in the disk controller could read and write data at a higher density, so that instead of 13 sectors (3.25 kB), 16 sectors (4 kB) of data could be stored per disk track, increasing the capacity from 113.75 kB to 140 kB per disk side — 16 kB of which was used by filesystem overhead and a copy of DOS, on a DOS 3.3-formatted disk, leaving 124 kB for user programs and data. DOS 3.3 was, however, not backwards compatible; it could not read or write DOS 3.2 disks. To address this problem, Apple Computer released a utility called "MUFFIN" to migrate Apple DOS 3.2 files and programs to version 3.3 disks. Apple never offered a utility to copy the other way. To migrate Apple DOS 3.3 files back to version 3.2 disks, someone wrote a "NIFFUM" utility. There were also commercial utilities (such as Copy II Plus) that could copy files from and to either format (and eventually ProDOS as well). Release 3.3 also improved the ability to switch between Integer BASIC and Applesoft BASIC, if the computer had a language card (RAM expansion) or firmware card.

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